Honeycomb structures are widely used as filters, catalyst carriers, or the like, in particular, as catalyst carriers in devices for purifying the exhaust gas of internal combustion engines such as automobile engines, and filters for purifying the exhaust gas of diesel engines.
When honeycomb structures are used as a carrier for a catalyst or the like in the devices for purifying automobile exhaust gas or the like, it is required to improve the purification performance of the catalysts for purifying exhaust gas in order to meet the exhaust gas regulation tending to be stepped up year by year in consideration of the environmental issues. On the other hand, the directivity toward high mileage and high output power is remarkably observed, and the reduction of the pressure loss is also required, in case of the catalysts for purifying exhaust gas in order to cope with such circumstances, from the viewpoint of the engine development.
Accordingly, in order to solve the above described problems, there is observed such an intensified movement that the partition wall thickness and the circumferential wall thickness of the honeycomb structures are made thinner so that the air permeability is improved and the pressure loss is thereby reduced, and simultaneously the exhaust gas purifying catalysts are reduced in weight and the heat capacities thereof are reduced, and the purifying performance in the warm-up period is thereby improved.
There are problems, due to this thinning mentioned above, that the partition wall strength and circumferential wall strength of the honeycomb structures are lowered, and, in particular, the wear (hereinafter referred to as erosion) of the opening end face exposed to the high-pressure exhaust gas discharged from an engine, namely, the wear of the partition wall end portions becomes severe.
As a measure to solve such problems, for example, JP-A-2000-51710 discloses a honeycomb structure provided with reinforced portions in the partition wall end portions and a production method thereof. In this laid-open specification, there is disclosed a honeycomb structure having an improved strength by thickening and densifying the partition walls, and coating and replacing the partition walls with a reinforcing material. Furthermore, as for the end portions of a honeycomb structure, it has been tried to make the partition walls thicker and denser around the end face by a method wherein a glass material such as glaze or water glass is applied onto the opening end face and firing is made, by a method wherein some particular components of the material composing a honeycomb structure such as cordierite are similarly applied and firing is made, and the like.
Although the above laid-open specification discloses that the end face strength against the pressure applied to the end face is improved by the end face reinforcement as described above, it is not disclosed to what extent the erosion resistance is improved. Moreover, it discloses roughly the relationship between the fundamental wall thickness of a partition wall and the wall thickness of the reinforced portion, however, it fails to carry out any investigation on the relation between the actual erosion resistance and the cell structure of the honeycomb structure, in particular, the cell density, the equivalent cell diameter, the total pore volume or the like; and hence there is a problem that the reinforcement effect is not fully exhibited or the pressure loss increase and heat capacity increase are brought about, depending on the cell structure of the honeycomb structure having the reinforcement applied.